This invention relates to a device for controlling an interelectrode gap in an electric discharge machining apparatus.
An electric discharge machining apparatus is well known in the art, in which an electrically conductive material such as metal is molten and machined by using high temperature energy which is generated by an electric discharge.
In the electric discharge machining apparatus, a pulse current is, in general, used as electrical energy. In order to maintain an electric discharge by using the pulse current, it is essential to suitably determine the distance between a tool electrode and a workpiece; that is; an interelectrode gap. The workpiece is partially molten and removed while being machined by the electric discharge. Therefore, in the case where the electrode and the workpiece are fixedly held, the interelectrode distance is gradually increased, as a result of which it becomes difficult to induce electric discharges, and finally the interelectrode distance is made so large that the electric discharge is stopped.
In order to overcome this difficulty; that is, in order to maintain the electric discharge constant, in general a method is employed in which, as the workpiece is machined, the electrode is moved towards the workpiece so as to maintain the interelectrode distance constant.
On the other hand, during an electric discharge machining, sludge is created between the electrodes (between the electrode and the workpiece) by the electric discharges, and usually it is washed away with a machining solution. However, in the case where the machining solution is not sufficiently supplied between the electrodes, or the interelectrode distance is small, often the electrodes are bridged with the sludge; that is, they are electrically short-circuited with the sludge. In this case, voltage high enough to induce electric discharge is not developed across the electrodes, as a result of which the electric discharge is stopped, or a large current is concentrated to damage the workpiece. In these cases, the interelectrode distance is increased to remove the part bridged or short-circuited by the sludge, or the machining solution flow path is improved.
As is apparent from the above description, an electric discharge machining operation is carried out by controlling (increasing and decreasing) the interelectrode gap; i.e., by maintaining the interelectrode distance substantially constant in average. The control of the interelectrode distance is a fundamental function which greatly affects the result of the electric discharge machining operation; i.e., the product in quality.
As was described above, the control and maintenance of the interelectrode distance are fundamental and essential for an electric discharge machining operation. However, it is considerably difficult to measure the interelectrode distance during electric discharge, and practice, it is impossible to do so. Consequently, the quantity of state substantially equivalent to an interelectrode distance is detected to thereby determine the interelectrode distance, and the interelectrode distance thus determined is compared with a value most suitable for the continuation of an electric discharge, for control of the interelectrode distance.
FIG. 2 is a block diagram showing one example of a conventional quantity-of-state detecting system.
In the detecting system of FIG. 2, an average value 10 of interelectrode voltages obtained from an interelectrode voltage waveform 8 is utilized for determination of an interelectrode distance. The method is so-called "an average voltage servo system". It is experimentally confirmed that the interelectrode voltage average value 10 is proportional to an interelectrode gap. Therefore, control is so made that, when the average voltage is higher than a target value, or specified value 1, the interelectrode distance is decreased so as to facilitate the occurrence of electric discharge, whereas when the average voltage is lower than the specified value 1, the interelectrode distance is increased so as to suppress the electric discharge. Thus, the electric discharge is maintained satisfactory by the control described above.
In the case where the interelectrode distance is increased because the system suffers from a disturbance due to a machining speed 5, a drive device is operated to move the electrode to maintain the interelectrode distance constant.
The increase of the interelectrode distance appears in the interelectrode voltage waveform through an interelectrode phenomenon; that is, electric discharges become rather difficult to induce. When electric discharges become rather difficult to induce, because of the reason indicated in FIG. 4 the interelectrode average voltage 10 is increased to a value 10b, thus differing from the specified value. The difference 11 therebetween, after being multiplied by a proportional constant 3, is supplied to a drive unit 4 as a signal for driving the drive unit 4. When the drive unit 4 feeds the electrode as much as the increase of the interelectrode gap, the interelectrode distance is set to the original value with which electric discharges occur satisfactorily, and the interelectrode average value is set to a value 10a; that is, it becomes equal to the specified value 1 again.
If, in the above-described operation, the proportional constant 3 is set to an excessively large value, the drive unit 4 will feed the electrode more than the increase of the interelectrode distance which has been caused by electric discharge machining, as a result of which the interelectrode distance is made shorter. In this case, the interelectrode average voltage is decreased, so that a signal to increase the interelectrode distance is applied to the drive unit 4. Whereby the electrode is moved to increase the interelectrode gap. However, in this case, the interelectrode average voltage 10 and the drive unit 4 are placed in an oscillation state, thus being much away from their ideal state. If, in contrast, the proportion constant 3 is excessively small, then a delay time required for restoring the system is increased, and it becomes impossible to quickly respond to the disturbance applied as a machining speed to the system. Thus, in this case also, it is difficult to maintain the electric discharge satisfactory.
As is apparent from the above description, it is necessary that the proportional constant 3, called "gain", is always set to the best value.
FIG. 3 is a diagram showing an electric circuit which practices the method and system described with reference to FIG. 2. In FIG. 3, parts corresponding functionally to those which have been described with reference to FIG. 2 are therefore designated by the same reference numerals of characters. In FIG. 3, reference numeral 20 designates a power source for supplying discharge energy; 21, a resistor for regulating the discharge energy as a current; 22, a switching element for forming a pulse current waveform; 23, an oscillator for the switching element; 24, an electrode; 25, a workpiece to be machined; 26, a machining vessel; and 27, a machining solution.
As is clear from the above description, in an electric discharge machining apparatus, in order to machine a workpiece with high efficiency, it is necessary to control at all times against disturbances such as the advancement of the machining operation and the formation of sludge which may adversely affect the interelectrode gap condition. For this purpose, it is essential to set to the best value the proportional constant, i.e., a gain for the drive unit adapted to change the interelectrode distance.
With the electric discharge machining apparatus, the best proportional constant, which varies depending on the machining speed, the machining current, the electrode area and the like is generally determined by the operator through his past experience or according to the machining conditions. Thus, the setting of the proportional constant is considerably difficult; however, it must be carried out frequently during machining operation.